UnityLaparoscopicSceneSimulator/.opencode/skills/pbr-materials-tissue/SKILL.md

name, description

name	description
pbr-materials-tissue	Author and tune PBR materials for the LaparoscopicSurgeryEmulation scene, focused on tissue / mucosa / cavity-wall / instrument surfaces. Use when creating, editing, or duplicating materials under Assets/Materials/LaparoscopicBenchmark/ or assigning them to renderers in the SurgeryBenchmark/ authored roots.

PBR Materials for Tissue (project-scoped)

This skill constrains material work in this project to a small, deliberate set of tissue-like PBR patterns. The available material inventory is fixed (see below); the goal is to tune them, not multiply them.

When to load

Load when the agent is about to:

• Create, edit, duplicate, or assign any .mat under Assets/Materials/LaparoscopicBenchmark/ or Assets/Materials/.
• Adjust shader properties on the URP/Lit, URP/SimpleLit, or the project's custom shaders (Assets/Shaders/RenderDepth.shader, Assets/Shaders/BlendShader.compute — the latter is a compute shader, not a material).
• Assign materials to renderers inside the SurgeryBenchmark/Shared/*Slot anchors or any scenario root.
• Diagnose "plastic-looking tissue", "mucosa too dry", "specular too harsh", or "normal map stretched" reports.

Material inventory (do not duplicate blindly)

Material	Purpose	Tunable axes
CavityWall.mat	The moist wall of the cavity	roughness, normal scale, subsurface tint, specular tint
Tissue_Default.mat	The "clean" tissue reference	roughness, normal scale, albedo tint, AO
Tissue_Wet.mat	Wet specular variant for Scenario_02	higher specular tint, lower roughness, slight clearcoat if URP/Lit
Tissue_LowTexture.mat	Low-texture variant for Scenario_03	uniform albedo, minimal normal, low contrast
InstrumentProxy.mat	The instrument placeholder	metalness 1.0, low roughness, neutral albedo

Scenario roots should reference one of these materials — do not create per-scenario material instances unless the change is bounded and reported. If a new variant is needed, add it to LaparoscopicBenchmark/ with a name that ends in the scenario tag, e.g., Tissue_Wet_S04.mat.

Available textures

• Assets/Textures/LaparoscopicBenchmark/TissueMucosa.png — the canonical mucosa PBR base map. Verify its import settings before assigning:
  • Texture Type: Default (or Albedo if no separate normal/roughness is bundled).
  • sRGB: on (it is colour data).
  • Alpha Source: None (unless a specific transparency case is approved).
  • Max Size: 1024 (or 2048 for hero work; do not exceed 2048 without a performance budget review).
  • Compression: High-quality BC7 / ASTC. Avoid DXT5 with alpha for opaque mucosa.
• Assets/Models/FabOrgans/StylizedDigestiveSystem_{Diffuse,Normal,Roughness}.png — the stylized organ set; usable for the AnatomySlot placeholder if needed.
• Assets/Models/organs/textures/ and Assets/Models/organs/archive.fbm/ — packed with archive.fbx. Do not move files out of Assets/Models/organs/ — the FBX references them by relative path.

URP/Lit PBR cheat sheet (defaults for tissue)

• Shader: Universal Render Pipeline/Lit (or Universal Render Pipeline/Simple Lit only as a performance fallback — never as the default for tissue).
• Surface Type: Opaque. Use Transparent only with explicit approval, and never for mucosa.
• Workflow: Metallic (this project is metallic workflow).
• Render Face: Front (tissue is single-sided).
• Double Sided: off by default; enable only on the cavity wall if it is a thin sheet viewed from inside.

Property	Tissue_Default	Tissue_Wet	Tissue_LowTexture	CavityWall	InstrumentProxy
Base color	warm pink-red #B66A6A ish	warm pink-red	uniform pink-red	deep red #7A2A2A ish	neutral metal #C8C8C8
Metallic	0.0	0.0	0.0	0.0	1.0
Smoothness	0.35	0.55	0.20	0.40	0.85
Normal Map	TissueMucosa normal (or the FabOrgans normal)	same	none / flat	cavity bump from TissueMucosa normal at low scale	none / flat
Normal Scale	0.5–1.0	0.5–1.0	n/a	0.3–0.7	n/a
Occlusion	AO from texture (or 1.0 flat)	1.0	1.0	AO from texture	1.0
Emission	off	off	off	off	off
Specular (workflow override)	leave default	leave default	leave default	leave default	n/a (metallic)

Treat the colour values as starting points to be confirmed by screenshot, not as final. The cavity is small and HDR-lit; hex picks rarely match what the eye sees.

Subsurface / translucency

URP/Lit does not have a first-class subsurface-scattering slot in 14.0.11. To get a mucosal feel:

• Keep Smoothness low enough (≤ 0.4) that the BRDF does not look like enamel.
• Drive the cavity / tissue color slightly redder than the reference photo; URP tone-mapping pulls warmth out.
• If translucency is required for backlit tissue (e.g., a thin membrane), use a Translucent Lit shader if the project has one, or a custom URP shader. Adding a new shader is a bounded change — state it in the handoff before writing the .shader file.

Roughness and specular realism

• Wet tissue in a laparoscope: Smoothness 0.5–0.6, Normal Scale 0.3–0.6, no Clearcoat.
• Dry / low-texture: Smoothness 0.15–0.25, no normal.
• Instrument proxy (steel): Metallic 1.0, Smoothness 0.7–0.9, slightly tinted toward warm grey to avoid chrome-looking plastic.

The "plastic" failure mode is almost always:

1. Smoothness too high (> 0.7) on tissue, or
2. Normal map stretched because UV scale does not match the mesh, or
3. Specular workflow left in default Metallic workflow but the colour was hand-tweaked to look right in the editor, breaking under tone-mapping.

UV scale and stretching

When a tissue-like material is applied to a primitive (Cube, Sphere, Cylinder, Quad), the default UVs produce obvious stretching. Mitigations:

• Use the same material on a mesh that has clean UVs (the FabOrgans or archive FBX meshes).
• For primitives, scale the UVs in the import settings of the mesh (read-only) or place a small tiling texture and adjust the material's Tiling (.x, .y) instead of stretching the UVs.
• A stretching texture is a visible failure, not a stylistic choice — fix it before reporting done.

Common operations and the right MCP call

Intent	MCP call	Notes
Read current material	manage_material(action="get_material_info", material_path="Assets/Materials/LaparoscopicBenchmark/Tissue_Default.mat")	Always re-read before editing
Set a single property	manage_material(action="set_material_shader_property", material_path="...", property="_Smoothness", value=0.4)	Property name is the shader's serialized name; for URP/Lit use _Smoothness, _BaseColor, _BumpScale, _Metallic
Set a colour	manage_material(action="set_material_color", material_path="...", property="_BaseColor", color=[0.71, 0.42, 0.42, 1.0])	Linear vs gamma: URP expects linear when colour space is Linear; pass sRGB-ish values and let the conversion happen if the project is Linear
Create a new variant	manage_material(action="create", material_path="Assets/Materials/LaparoscopicBenchmark/Tissue_Wet_S04.mat", shader="Universal Render Pipeline/Lit")	Then set properties; do not assign to a renderer until verified
Assign to a renderer	manage_material(action="assign_material_to_renderer", material_path="...", target="Tissue_S04_00", search_method="by_name", slot=0)	Always re-read after assigning
Tint a renderer in-scene without changing the asset	manage_material(action="set_renderer_color", target="...", color=[...], mode="property_block")	Property block is reversible and does not duplicate the asset

Reversibility checklist

1. Did you read the material's current state before editing? If no, do that first.
2. Did you change the shared asset (Tissue_Default.mat etc.) rather than create a per-instance override? Document the choice in the handoff.
3. Will any scenario that currently uses this material be affected? If yes, list the affected scenarios in the handoff.
4. Is the change expressed as a single property tweak, or a coordinated multi-property tweak? Coordinated is fine, but state the target look in one sentence.
5. After the change, does the screenshot show a believable tissue surface, or does it look like plastic / wax / enamel? If plastic, lower Smoothness first; do not increase emission.

Anti-patterns to refuse

• Creating per-scenario .mat files for every minor variation — tune the existing five.
• Setting Metallic > 0 on tissue or cavity wall. Tissue is not metal.
• Setting Emission > 0 on tissue. If a region "needs to glow", it is probably a lighting problem, not a material problem.
• Using Standard shader (Built-in pipeline) on URP — pink materials are a URP/Built-in mismatch symptom.
• Stretching UVs by scaling the GameObject instead of adjusting material tiling.
• Generating textures procedurally inside the project (no procedural assets without approval).
• Renaming any of the existing Tissue_*.mat, CavityWall.mat, InstrumentProxy.mat — the names are part of the contract referenced by LaparoscopicSurgeryEmulation.md.